Hello:
I take 600 mg of Lithium also. To share my experience-I did gain about 15lbs initially.
Lithium binds to fat(don't know the technology behind it) and I experienced a lot of fat around my middle area that I never had. So hated that.
So now I have taken off about 10lb, but have to really monitor my diet(about 500 cal less a day than I used to) and do intense cardio-boot camp, kickbox, lift weights, 6 days per week.
Eating a low fat, high protein diet has worked for me. It is quite a struggle.
However, for me, the mood stabilizing that has happened has been huge. Changed my life.
Give Lithium a chance, it may work well for you, and if so you will be willing to do the work to keep the weight off.
Just keep an eye on the weight gain and try to take steps accordingly.